1. Field of the Invention
The present invention relates to an optical pickup having a liquid crystal panel for correcting an aberration.
2. Description of Related Art
As to an optical pickup, there is a known technique of mounting a liquid crystal element such as a liquid crystal panel thereon in order to correct various aberrations caused by inclination of an optical disc (also referred to as “disc tilt”), a difference in thickness between different optical discs such as a CD and a DVD and so on. For instance, in the case of the optical pickup described in Japanese Patent Application Laid-Open No. 10-289465, an aberration correction (a so-called tilt correction) to the inclination of the disc is performed by providing wavefront aberration correcting means comprised of the liquid crystal panel on a light path between a laser source and an objective lens. The liquid crystal panel as the wavefront aberration correcting means has its electrodes divided into forms corresponding to wavefront aberration distribution caused by a tilt angle of the optical disc. Different voltages are applied to the divided portions to give different refractive indexes, and a phase difference is given to a laser beam so as to correct the wavefront aberration. To the liquid crystal panel, the voltage corresponding to a direction and an amount of the aberration to be corrected is applied in reference to a predetermined bias voltage. Generally, the bias voltage is set at a substantially midpoint of a dynamic range of the phase difference obtained by the liquid crystal panel so that the aberration of both directions can be corrected.
At the time of using the liquid crystal panel for the aberration correction in the optical pickup having two or more wavelengths, when the same bias voltage is set to the laser beams having different wavelengths, the correction voltage applied to the liquid crystal panel for the aberration correction and the phase difference (i.e., a sensitivity of the phase difference) accordingly generated by the liquid crystal panel are sometimes different for each wavelength. Concretely, the sensitivity of the phase difference of the long wavelength becomes low, and the sensitivity of the phase difference of the short wavelength becomes high. The sensitivity of the phase difference indicates a rate of a variation of the phase difference in correspondence with a variation of the applied correction voltage. Therefore, between the optical discs using different wavelengths, since the correction voltages to be applied to the liquid crystal panels in order to correct the same disc tilt amount are greatly different, it is necessary to widely maintain the dynamic range of the control voltage. In addition, even if the dynamic range of the control voltage is widely maintained, since the resolution of the control becomes low in the wavelength having the high sensitivity of the phase difference, it is problematic that the control of the liquid crystal panel cannot be accurately executed.
Japanese Patent Application Laid-Open No. 2001-273663 discloses a method of switching the bias voltage applied to the liquid crystal element in an aberration correction apparatus using the liquid crystal element.